Introduction
Biomass is the most abundant renewable resources on earth, and is considered to be a promising substitute for fossil resources due to its vast concentration of carbon and hydrogen.1 As the second largest component of biomass, lignin is presently underutilized due to its structural complexity, poor solubility and high bond dissociation energy.2 Conversely, cellulose and hemicellulose are used widely for the production of bio-ethanol and sugars industrially.3 At present, 50-70 million tons of lignin is produced annually from pulping activities, but only ~2% of it is commercially utilized as value-added materials, such as water-reducing admixtures or as a surfactant,etc .4 On the other hand, it is regarded as the only renewable source of key- and high-volume aromatic polymers, making it a potential green precursor for the production of aromatic products usually refined from petroleum.5,6 Therefore, the development of efficient technologies for the production of value-added chemicalsvia lignin depolymerization is not only environmentally benign, but also meets the requirements for sustainability. To the best of our knowledge, numerous and different strategies have emerged in recent years. For example, the addition of formic acid significantly promotes the yield of aromatics during the depolymerization of oxidized lignin, giving rise to a value of the yield over 60wt. %.7Moreover, treatment of lignin with formaldehyde protects side-chain hydroxyl groups, allowing for near-theoretical yields of aromatic products by hydrogenolysis.8 Recently, the selective production of diethyl maleate (DEM) was achieved by using polyoxometalate ionic liquid catalysts (POM ILs) to promote the selective oxidation of lignin coupled with esterification of the resulting aromatic monomers.9 Nevertheless, these processes still exhibit some drawbacks, for instance, high temperature, high H2 pressure and long reaction time plague the heterogeneously catalyzed processes to realize both high lignin conversion and product yields, largely due to the poor contact between the macromolecule and catalyst. On the other hand, product separation and isolation can prove quite energy consuming and costly for systems employing homogeneous catalysts.10 Generally, process intensification and coupling techniques are a potential alternative to solve these problems,11,12 among which the emulsion approach has received a resurgence in interest recently. Because the emulsion system is able to provide a much larger interface for reactions involving in problem related with the incompatibility of reactants.13
As described above, lignin can be employed as a surfactant,4,14,15 or as a precursor for the production of functional surfactants due to the numerous hydrophilic and hydrophobic groups it contains.16 Hence, developing an emulsion system utilizing lignin as the surfactant has obvious advantages, and several new emulsion systems for lignin depolymerization have been demonstrated on the basis of its self-surfactivity. For example, over three times of phenolic monomers’ yield has been obtained in a water/oil (W/O) emulsion reactor comparing to those in a standard solvent system.17 Basing on this work, a purpose-designed emulsion was utilized for the depolymerization of lignosulfonate to result in the generation of appreciable yields of phenolic monomers and 4-ethyl guaiacol, 116.1 and 39.3 mg g-1respectively.18 Furthermore, Wessling et al.proposed an emulsion system comprised of a deep eutectic solvent and an extractant for electrochemical oxidation of kraft lignin to low molecular weight products (ranging from 100 to 600 Da).19 The above examples make it clear that emulsion approaches can promote lignin depolymerization to some degree, yet higher lignin conversion facilitates a destabilization of the system and thus reduces process efficiency, although the demulsification can be seen as a kind of benefit due to the partitioning effect achieved after reaction.
To overcome the above difficulties, a surfactant-free microemulsion (SFME) system could be a better choice for the intensification of lignin depolymerization, due to its thermodynamic stability and much more larger interfacial area.20 The SFME is more commonly referred to as a pre-Ouzo or detergentless microemulsion, so-called due to the spontaneous formation of a stable emulsion with only the addition of water, a property that is shared by Ouzo, a famous alcoholic beverage in Greece.21,22 Thus, SFMEs are known to have properties similar to those of surfactant-based microemulsions (SBMEs), providing both the solubilization effect and capacity to dissolve the immiscibility solvents,23 but not need to separate the surfactant from the system, showing great advantage both in cost and process.24,25 Herein, a novel oil/water (O/W) SFME containing octane, n -propanol and water was proposed after carefully screening, and the oxidative depolymerization of lignin in this SFME was conducted basing on the construction of the ternary diagram and the determination of lignin solubility distribution in its different subregions. Experimental results showed that around 40 to 500wt. % increase of phenolic monomers in SFME reactor was achieved with comparison to those in non-microemulsion systems, illustrating great potential to develop novel SFME for the valorization of biomass.